In addition to their role as protein monomeric units, amino acids are energy metabolites and precursors of many biologically important nitrogen-containing compounds, such as heme, physiologically active amines, glutathione, other amino acids, nucleotides, and nucleotide coenzymes. Excess dietary amino acids are neither stored for future use nor excreted. Rather they are converted to common metabolic intermediates such as pyruvate, oxaloacetate and alpha-ketoglutarate. Consequently, amino acids are also precursors of glucose, fatty acids and ketone bodies and are therefore metabolic fuels.
Amino acid decarboxylases are induced in cells as a response to various forms of stress. In Salmonella typhimurium lysine decarboxylase (EC 4.1.1.18) is induced by low pH, is required for acid tolerance, and contributes significantly to pH homeostasis in environments as low as pH 3.0 (Park et al. (1996) Mol. Microbiol. 20:605-611). At least two different lysine decarboxylases exist in Escherichia coli: an extensively characterized inducible decarboxylase, and a decarboxylase which is present in low amounts upon derepression by an as yet undetermined factor (Lemonnier and Lane (1998) Microbiology 144:751-760). A monomeric lysine decarboxylase from soybean has been purified and characterized, but its sequence has not yet been determined (Kim et al. (1998) Arch. Biochem. Biophys. 354:40-46).
Two similar but distinct enzymes are referred to as dopa decarboxylase:tyrosine decarboxylase and tryptophan decarboxylase. Dopa decarboxylase:tyrosine decarboxylase has been shown to be involved in several different pathways such as histidine metabolism, tyrosine metabolism, tryptophan metabolism, phenylalanine metabolism, and alkaloid biosynthesis. In the eastern tiger swallowtail butterfly Papilio glaucus dopa decarboxylase:tyrosine decarboxylase provides dopamine to the two major color pigments, papiliochrome (yellow) and melanin (black). Dopa decarboxylase:tyrosine decarboxylase activity is spatially and temporally regulated, being utilized early in presumptive yellow tissues and later in black, forming part of a developmental switch between yellow or black (Koch et al. (1998) Development 125:2303-2313).
L-tyrosine decarboxylase (EC 4.1.1.25) is involved in an early, and potential rate-limiting step, in the biosynthesis of isoquinoline alkaloids, such as morphine and codeine, in opium poppy (Papaver somniferum). This enzyme catalyzes the conversion of L-tyrosine to tyramine and carbon dioxide. Several members of the tyrosine decarboxylase family, differentially expressed in various tissues, have been identified in poppy (Facchini and De Luca (1994) J. Biol. Chem. 269:26684-26690). Four parsley (Petroselinum crispum) tyrosine decarboxylases have been identified from cDNAs representing genes that are transcriptionally activated upon fungal infection or elicitor treatment. The deduced protein sequences share extensive similarity with two functionally related enzymes, tryptophan decarboxylase from periwinkle and dopa decarboxylase:tyrosine decarboxylase from Drosophila melanogaster (Kawalleck et al. (1993) J. Biol. Chem. 268:2189-2194).
Tryptophan decarboxylase (EC 4.1.1.28) catalyzes a key step in the biosynthesis of terpenoid indole alkaloids catalyzing the conversion of tryptophan to tryptamine and carbon dioxide. Chimeric gene constructs in which a tryptophan decarboxylase cDNA is linked in the sense or antisense orientation to the cauliflower mosaic virus 35S promoter and terminator have been expressed in callus and cell suspension cultures. Calluses harboring the tryptophan decarboxylase sense construct showed increased levels of tryptophan decarboxylase protein and activity, as well as the tryptamine content, but no significant increase in terpenoid indole alkaloid (Goddijn et al. (1995) Transgenic Res. 4:315-323). Tryptophan decarboxylase supplies tryptamine for the indole moiety of Camptothecin, a valuable anti-cancer monoterpene alkaloid, and its derivatives. Tryptophan decarboxylase is considered a key step in monoterpene indole alkaloid biosynthesis as it links primary and secondary metabolism. Two autonomously regulated tryptophan decarboxylase genes from Camptotheca have been identified and isolated. One of these genes is part of a developmentally regulated chemical defense system while the other gene serves as part of a defense system induced during pathogen challenge. When expressed in Escherichia coli, the product of each gene will decarboxylate tryptophan, but is inactive against tyrosine, phenylalanine and 3,4-dihydroxyphenylalanine (dopa) (Lopez-Meyer and Nessler (1997) Plant J. 11:1167-1175).